Process for the electrochemical manufacture of silver containing catalysts

ABSTRACT

A process is disclosed whereby a silver containing catalyst is produced, in a particle size ranging from 300 to 1,500 A, through the pulsed electrolysis of a solution of a silver salt (e.g., silver nitrate) in the presence of a complexing agent (e.g., ammonia) wherein current is fed to the solution for spaced periods of a few seconds duration. Advantageously, after about 10-15 cycles of feeding and interrupting the current to the solution, a current inversion lasting from a few seconds is effected. The silver is obtained in the form of a powder which, after washing, can be used directly or supported on a ceramic material as catalyst for the production of ethylene oxide.

United States Ptent Rivola et al.

[ PROCESS FOR THE ELECTROCHEMICAL MANUFACTURE OF SILVER CONTAINING CATALYSTS Inventors: Luigi Rivola; Vittorio Mormino;

Bruno Notari, all of San Donato, Milanese, Italy Snam Progetti S.p.A., San Donato, Milanese, Italy Filed: July 28, 1972 Appl. No.: 275,937

US. Cl. 204/10, 204/228 Int. Cl C22d 5/00, BOlk 3/00 Field of Search 204/10, 91, 109, 228

[56] References Cited UNITED STATES PATENTS 2,216,167 l0/l940 Fisher ..204/10 1 Sept. 17, 1974 3,463,711 8/1969 Geyken ..204/109 FOREIGN PATENTS OR APPLICATIONS 197,980 10/1967 U.S.S.R 204/10 129,447 1/1960 U.S.S.R ..204/109 Primary ExaminerT. M. Tufariello Attorney, Agent, or Firm-Ralph M. Watson, Esq.

[5 7 ABSTRACT A process is disclosed whereby a silver containing catalyst is produced, in a particle size ranging from 300 to 1,500 A, through the pulsed electrolysis of a solution of a silver salt (e.g., silver nitrate) in the presence of a complexing agent (e.g., ammonia) wherein current is fed to the solution for spaced periods of a few seconds duration. Advantageously, after about l0-l5 cycles of feeding and interrupting the current to the solution, a current inversion lasting from a few seconds is effected. The silver is obtained in the form of a powder which, after washing, can be used directly or supported on a ceramic material as catalyst for the production of ethylene oxide.

13 Claims, 5 Drawing Figures PAIENM SEP 1 7 1924 mm 3 UF 5 FIG. 3

Pmminsmmu SHEET 5 OF 5 LNNP m Pm? v Pm? UNNP PROCESS FOR THE ELECTROCHEMICAL MANUFACTURE OF SILVER CONTAINING CATALYSTS The present invention refers to the electrochemical manufacture of silver containing catalysts.

More particularly the present invention is related to the manufacture of silver containing catalysts useful for the catalytic production of ethylene oxide.

Many processes are known for the manufacture of silver containing catalysts for the ethylene oxide production; on the other hand it is also known that the silver structure and the operative conditions used for its preparation are very important in order to obtain a high activity and selectivity. The chemical methods which have been proposed up to now essentially involve the silver salt or compound decomposition to obtain finely divided silver. The above methods present the main drawback of the poor reproducibility and inevitable losses of silver which can arise in the course of treatment, therefore expensive processes of recovery are required.

It should be possible to produce silver with high yields and in a reproducible way by using electrochemical deposition processes of said metal; however it is also known that the electrolysis of silver solutions yields the production of metal having a compact structure or in any case a high particle size which does not allow such an activity and selectivity to be employed in an industrial process for producing ethylene oxide by ethylene oxidation.

It has been found that it is possible to produce by electrochemical way silver catalysts having particle size lower than 1,500 A. and preferably comprised between 300 A. and 1,500 A. which, when employed in the ethylene oxide synthesis by ethylene oxidation, give high activity and selectivity.

The process according to present invention for producing silver catalysts comprises a pulsed electrolysis of silver salt solutions in presence of complexing agents.

With the expression pulsed electrolysis it is meant an electrolysis wherein the current supply at intervals is broken off.

This breaking off the current can be in some case followed by an inversion of the supplying of the current.

In a non-limitative sense the periods of feeding the current through the solution can be comprised between 3 and seconds followed by breaking off periods comprised between 3 and 60 seconds; possible after 10-15 cycles of supplying and breaking off the current a current inversion can be effected for a period preferably comprised between I and 60 seconds.

The solutions of silver salts advantageously used according to the invention can consist of silver nitrate, silver chloride, silver sulphate, silver acetate and silver.

oxalate which are complexed with ammonia.

Preferably the concentrations of said solutions of silver salts can be comprised between 0.1 g and 10 g of silver per liter of solution.

The amount of complexing agent is preferably comprised between 3 and 50 moles per atom gram of silver employed.

It is preferred to employ also a buffer solution to keep constant the value of pH of the electrolytic solution.

The buffer solutions which can be advantageously used are the ones which keep the value of pH between 9 and 12.5. Example of said buffer solutions are: glycine sodium hydroxide, disodic phosphate sodium hydroxide and the like; very good results have been obtained with the mixture borax sodium hydroxide.

The electrolysis is advantageously carried out at a temperature comprised between 0 and C and preferably between 10 and 40C.

The potential can be kept between 500 and l ,500 mV measured with respect to the saturated calomel electrode. The apparent current density can be com prised between 0.1 and 0.5 amp/cm and preferably between 0.2 and 0.3 amp/cm The electrolytic cell anode can be constituted by graphite, platinum, platinumrhodium, titanium and in a general sense any good conductor which cannot be attacked by alkali material while the cathode can be constituted by silver, stainless steel, graphite and generally the same materials used as anode.

Preferably during the electrolytic deposition thesilver salt solution is kept under vigorous stirring. The obtained silver, in form of a powder, after washing can be directly used, as it is or preferably supported on a ceramic material, as catalyst for the production of ethylene oxide.

In the accompanying drawings some kind of devices to carry out the process according to the invention are shown which are not to be intended as limitative thereof.

FIG. 1 represents an electrolytic cell for the silver deposition according to the invention.

FIG. 2 shows the scheme of a unit for the production of silver on an industrial scale.

FIGS. 3 and 4 represent respectively a sectional view and a plan view of the industrial unit of FIG. 2.

FIG. 5 shows the current supply scheme of the cell according to FIGS. 3 and 4.

In FIG. 1, reference 1 indicates the circulating pump of the electrolyte, 2 is the anode, for instance of graphite, 3 is the cathode, for instance of silver net, 4 is the very cell, wherein the electrolysis takes place, whose bottom consists of the silver net, 5 is the electrolyte contained in the reservoir 11, 6 is a voltmeter to measure the direct tension supplied to the cell, 7 is a timer for the inversion of the cell, 8 is a direct current generator, 9 is an amperometer for measuring the direct current supplied to the cell, 10 is a timer for the pulsed electrodeposition.

The working of the cell is the following: the electrolyte, prepared aside, in a container not shown, kept into the reservoir 11 and drawn by pump 1, is introduced in the cell from the top thereof. The electrolyte is periodically renewed when its silver content decreases under predeterminated values.

The cell is supplied with direct current by a generator 8 through a timer 10 which has the function of sending to the cell itself current impulses; periodically the sense of the current is inverted by the timer 7. The electrodeposited silver detaching from electrode 3 falls directly in the reservoir 11 through the opening in the bottom of the cell.

In FIG. 2 the reference 101 represents a continuous centrifuge, 102 is the electrolytic cell provided with a stirrer 108 and a circulation pump 106, 103 and 104 are storing reservoirs of the silver salt solution which are equipped with a circulating pump 107 and from 105 the deposited silver is recovered.

The working is as follows: in the electrolytic cell 102, details of which shall be found in the following description with reference to FIGS. 3 and 4, electrolytic silver is deposited which, as a suspension, is fed by means of the pump 106 to the centrifuge 101 and discharged through the duct 105. The pump 106 has also the function of recycling the electrolyte by a suitable series of valves, the electrolyte is fed to cell 102 from reservoirs 103 and 104 by means of the pump 107.

The reservoirs 103 and 104 work alternatively, i.e. one is used for preparing the electrolyte while the other, containing the electrolyte, feeds the cell. Naturally in said reservoirs stirring means are provided which have not been shown. The details of the electrolytic cell are reported in FIGS. 3 and 4 wherein 109 shows the anode, for instance perforated graphite, 110 is the cathode, for instance of silver net, 111 are through terminals for feeding the electrodes with direct current, 112 is the valve which allows the output toward the centrifuge 105, 113 is the recycling duct to the centrifuge, 114 is the recycling duct to the cell, 115 is the feeding duct of fresh solution.

From FIG. 4 it appears that the anode-cathode pairs are connected each other in series by group: in the drawing the groups comprise not restrictively five pairs.

The electric scheme for feeding the industrial cell is diagramatically represented in FIG. 5.

As appears in FIGS. 3 and 4 the 30 electrodes are fed in small independent groups of five pairs in series.

The feeding with direct current (which is supplied by impulses of 5 see.) is supplied by an amperostatic generator 123 which feeds, by means of the commutation units 121a to 121d, only one small groups, respectively 122a to 122f of five pairs of elements each and one after the other.

Summing up at the start the commutation 121a feeds only the small group 122a for the time of the impulse, thereafter the feeding of the small group 122 stops and the one of the small group 122 is started by the commutation unit 121b for the same time and so on until the last small group.

The feeding cycle starts again as programmed by the logical control unit 120, consisting of a square waves generator 116, a frequency divider 117, a counting unit in binary code 118 and a binary-decimal unit 119.

Periodically the sense of the feeding direct current of the single small groups 122a to l22f is inverted by means of the inversion unit 124 which directly operates the amperostatic 123.

The following examples illustrate the invention but are not to be'inte'nded as limitative thereof.

EXAMPLE 1 100 g of silver nitrate 750 g of borax (Na B O 10H O), 400 g of sodium hydroxide and 10 g of presolubilized carboxymethylcellulose, are dissolved in distilled water up a total volume of solution of 100 liters, 600 cc of Nl-I in a water solution are added to the above solution.

Said solution containing silver as an ammonia complex is utilized as an electrolyte in an electrolytic cell (shown in FIG. 1), using silver net as cathode (1 X 1 mm mesh) and tubular or perforated plate graphite as anode. The electrical contacts are made with gold wire which in the operative conditions of the cell behaves as an inert metal.

The solution is circulated in the sense directed in FIG. 1 by means of an external circulating pump and the silver content is kept constant by subsequent additions of silver nitrate in an ammonia solution.

The continuous feeding current of the cell is delivered by impulses having a rectangular profile of a period of 10 sec.

The cell feeding circuit is arranged in such a way to give 10 sec. of delivering and 60 sec. of rest.

Every 6-12 cycles the sense of the current is inverted in order to make easier the detachment of the silver deposited on the cathode.

The cathodic deposition is carried out at constant current (amperostatic). The main anodic process is the discharge of oxygen, the main cathodic process is the discharge of silver.

For this reason the electrolytic solution depletes of silver ion (ammonia complexed) and concentrates of ammonium nitrate. V

The operative conditions are the following:

temperature apparent cathodic surface apparent cathodic current density discharge tension of Ag. after deducted the ohmic drops apparent anodic surface electrolyte flow through the cathode The total tension feeding the cell is comprised between 7.5V and 15V. The faradic efficiency of the produced silver (cathodic) is comprised between and percent.

The silver content is kept constant by adding every 2 hours 2.0 to 2.4g of ammonia complexed silver nitrate.

The total consumption of electric power is therefore comprised between 2.3 and 4.5 kwh per kg of produced silver.

The silver powder, obtained by the method according to the invention was partially recovered from the solution and partially from the surface of the electrode, filtered, washed with distilled water and then dried in an oven for 3 hours.

The productivity has been raised up to 20-24g of silver for 24 hours of continuous working, i.e., of about 2g each day for cm of apparent cathodic surface. With the silver produced according to the above mentioned process a test of catalytic activity has been carried out.

The silver has been deposited on a ceramic carrier and the so obtained catalyst containing 15 percent of Ag has been introduced in a tube of 2.4 cm of diameter equipped with an external jacket for the circulation of a thermostating liquid. The height of the catalyst bed was about 1 meter. The reactor has been fluxed at atmospheric pressure with a gaseous mixture having the following composition: ethylene 5%, CO 6.5%, 0 5%, N 83.5%. The feeding flow was 2l0Nl/h and the contact time about 4.6 sec. At 200C. a good conversion of ethylene to ethylene oxide was obtained with a selectivity (an ethylene oxide moles per moles of reacted ethylene) of 80/, with a conversion of 25 percent.

EXAMPLE 2 In this example one of the possible industrial process is shown for the production of a silver based catalyst by pulsed electrochemical deposition, which uses silver nitrate as starting material and a particular electrolytic cell. A solution having the following composition:

Ag No, 0.5 5g/l Borax 5 20g/l Carboxymethylcellulose 1 0.5g/l Ammonia SOg/l NaOH 3.5 g/l was employed as electrolyte in an electrolytic cell illustrated in FIGS. 3 and 4 using as a cathode silver net (mesh with a size of 1 X 1 mm) and as anode perforated compact graphite. The complete scheme of the unit is shown on P16. 3. The solution was kept under continuous stirring by means of a mechanical stirrer 108 and passes through the electrodes which as apparent on FIGS. 3 and 4 were disposed radially in the bottom of the cell. The feeding current of the electrolytic cell was delivered as impulses having a rectangular profile of 5-15 sec. The comprehensive cycle consisted therefore of 5-15 see. of feeding and 60 sec. of rest. Every 10-20 cycles the electrical current sense was inverted in order to allow the detachment of the silver deposited on the cathode.

The total current for any single impulse was 700-900A.

The operative conditions of this cell are as follows:

temperature total apparent cathodic surface single elements of 60 X 60 cm) cathodic current density discharge tension of Ag after deducted the ohmic drops 1'00"] temperature 10 m 0.2-0.3 A/crn" 40-1000 rounds/minute The main anodic process is the discharge of oxygen, the main cathodic process is the discharge of the silver.

The faradic efficiency of the discharge of Ag is 80 90 percent and the tension of the comprehensive feeding is comprised between 5 15 V.

The consumption of electric power is therefore comprised between 1.5 and 4.5 kwh per kg of produced silver.

Since the main anodic product is oxygen, the solution of example 2, shall more and more depleted of Ag and on the contrary shall concentrate of NH NO Therefore a feeding system is provided to introduce fresh solution continuously recycled and adjusted as to the Ag content with a highly concentrated silver nitrate ammonia solution (see diagram of FIG. 3). The solution is deemed no more useful when the ammonium nitrate concentration (in moles) became 100 times higher than the one of silver nitrate (in moles).

From this exhaust solution, silver was recovered by means of a second electrolytic cell substantially identical to the one shown on FIG. 1.

The amount of silver to be recovered is about 1 percent with respect to the amount of produced silver.

The higher productivity of this electrolytic cell, for the catalytically active silver is of 300kg/day for a continuous working of 24 hours (by using therefore an apparent electrodic solution of 10 mm' for a total yearly production of more than t/year of metallic silver.

The produced silver, used as a catalyst according to the modalities of example 1 gave values of conversion and selectivity in the ethylene oxide manufacture practically identical to the ones of the preceding examples.

What we claim is:

1. Process for producing, by electrochemical deposition, silver catalysts consisting of particles smaller than 1,500 A. in size, wherein a solution of silver salt is subjected to a pulsed electrolysis in an electrolytic cell by periodically feeding current to the silver salt solution in a single direction and then breaking off the feeding of the current through a series of pairs of cycles and, between each pair of cycles, feeding current to the silver salt solution in the reverse direction.

2. Process according to claim 1 characterized in that the period of feeding the current in said single direction is between 3 and 10 seconds.

3. Process according to claim 1 wherein the period of breaking off the feeding of the current is between 3 and 60 seconds.

4. Process according to claim 1 characterized in that the direction of the feeding of the current is inverted every 10-50 cycles of feeding and breaking off the current.

5. Process according to claim 4 characterized in that the period of feeding the current in the reverse direction is between 1 and 60 seconds.

6. Process according to claim 1 characterized in that the solution of silver salt is a member of the group consisting of silver nitrate, silver chloride, silver sulphate, silver acetate and silver oxalate in the presence of a complexing agent.

7. Process according to claim 6 characterized in that the complexing agent is ammonia.

8. Process according to claim 6 characterized in that the concentration of the silver salt solution is between 0.1 and 10g of silver per liter of solution.

9. Process according to claim 7 characterized in that the amount of complexing agent is between 3 and 50 moles per gram atom of used silver.

10. Process according to claim 1 characterized in that a buffer solution is employed which keeps the pH of the silver salt solution between 9 and 12.5.

11. Process according to claim 1 characterized in that the electrolysis is carried out at a temperature between 10 and 40C.

12. Process according to claim 1 characterized in that the potential of said current is between 500 and 1,500 mV with respect to the saturated calomel electrode.

13. Process according to claim 1 characterized in that the current density is between 0.1 and 0.5

amp/cm and preferably between 0.2 and 0.3 amp/cm? 

2. Process according to claim 1 characterized in that the period of feeding the current in said single direction is between 3 and 10 seconds.
 3. Process according to claim 1 wherein the period of breaking off the feeding of the current is between 3 and 60 seconds.
 4. Process according to claim 1 characterized in that the direction of the feeding of the current is inverted every 10-50 cycles of feeding and breaking off the current.
 5. Process according to claim 4 characterized in that the period of feeding the current in the reverse direction is between 1 and 60 seconds.
 6. Process according to claim 1 characterized in that the solution of silver salt is a member of the group consisting of silver nitrate, silver chloride, silver sulphate, silver acetate and silver oxalate in the presence of a complexing agent.
 7. Process according to claim 6 characterized in that the complexing agent is ammonia.
 8. Process according to claim 6 characterized in that the concentration of the silver salt solution is between 0.1 and 10g of silver per liter of solution.
 9. Process according to claim 7 characterized in that the amount of complexing agent is between 3 and 50 moles per gram atom of used silver.
 10. Process according to claim 1 characterized in that a buffer solution is employed which keeps the pH of the silver salt solution between 9 and 12.5.
 11. Process according to claim 1 characterized in that the electrolysis is carried out at a temperature between 10* and 40*C.
 12. Process according to claim 1 characterized in that the potential of said current is between -500 and -1,500 mV with respect to the saturated calomel electrode.
 13. Process according to claim 1 characterized in that the current density is between 0.1 and 0.5 amp/cm2 and preferably between 0.2 and 0.3 amp/cm2. 